My central air unit hasn't been producing very cold air, so I just had a local place come check it out. He said the head pressure was jumping up and down and it was making a sound that sounded like air got in the lines? Is this possible and if so how did it happen?

Non condensible gases could only get in your closed system if they were introduced by a technician when he was changing the unit or adding freon. If things are done properly this won't happen. Sounds as though you might have non condensables or a restrictions could be a tough fix or could be easy.

Well, right now he is in the process of removing all the old refrigerant and replacing it. He said that was the only way to fix it. I had to have the system charged a few summers ago, but other than that it hasn't been worked on, so I'm not sure how the non condesables would have gotten in there. Looks like its gonna be a spendy day.

Forgive my skepticism, but when someone tells me my head pressure is jumpng all over, I would ask, "May I please see it!!". He has the gauges there and can easily show you so you can see how much the pressure is fluctuating. You have a perfectly well running unit... leave well enough alone unless you have true cause. Pressures can fluctuate for many reasons and if it's a few PSI, that's normal.

i don't view fluctuating pressures as normal. in the case of air, the discharge pressure is going to fluctuate, possibly wildly. the overall discharge pressure will be abnormally high. air and water are considered to be 'non-condensables' in a refrigerant system. one way air can get in is if there is a leak on the low pressure side of the system that causes the system to drop into a vacuum while running. the air sucks in through the leak. if you called the tech because it wasn't cooling and he saw it was low and added refrigerant, then saw the fluctuation, that makes sense. if this is a new problem, that is the only explanation. if this is the case, the tech needs to leak check your system, seal the leak, replace the liquid line filter drier, evacuate and recharge. removing the refrigerant with the air in it is the correct procedure, but make sure the leak is identified and dealt with. otherwise, you will have a repeat performance.